Many reactions in organic chemistry are accelerated or in some instances even made possible by irradiation with visible or ultra-violet (UV) radiation. Such reactions include, for example, fission reactions, e.g., in the vitamin D series, rearrangement reactions, e.g., cis-transisomerizations, and addition reaction, e.g., of maleic acid to benzene. However, the technically most important addition reactions of this kind are the photochemically initiated polymerization reactions.
For all of these reactions, it is necessary that at least a portion of the reaction mixture be capable of absorbing irradiated visible or UV radiation. When this portion is one of the reaction components, the reactions can be carried out without further additives simply by appropriate irradiation of the reaction mixture. However, frequently the reaction components are not able to absorb a sufficient amount of the photochemically effective irradiation. In such cases, substances called photosensitizers are frequently added. These do not participate in the reaction but are able to absorb the visible UV irradiation and to transfer the absorbed energy to one of the reaction components. Important criteria for the choice of such sensitizers are, inter alia, the nature of the reaction to be carried out, the relationship of the absorption spectrum of the sensitizer to the spectral energy distribution of the available source of radiation, the solubility of the sensitizer in the reaction mixture and the influence on the end product of residues of the sensitizer and/or the products resulting therefrom during the photochemical reaction.
As sensitizers for the photopolymerization of unsaturated compounds, previously there have principally been used benzophenone derivatives, benzoin ethers, benzil monoacetals and .alpha.-haloacetophenone derivatives. However, these substances possess various disadvantages which distinctly limit their industrial usefulness. Such disadvantages include, in particular, the tendency of monomers of prepolymers to polymerize when mixed with photosensitizers of these groups prior to irradiation, i.e., in the dark. Consequently, many reaction mixtures containing such sensitizers possess only a low stability in the dark. Other compounds from these classes possess only a low chemical stability; thus, for example, some benzil monoacetals are split even by very small amounts of water, e.g., by atmospheric humidity, into benzil and alcohol. Furthermore, others of these known sensitizers cause a yellowing of the resultant photopolymerized polymers, which, especially in the case of normally colorless synthetic resins or in particular in the case of UV-hardened printing dyes, is highly undesirable. For this last-mentioned field of use, the generally low solubility of the known sensitizers in the monomers of prepolymers often plays an important role. Since, as a rule, printing dyes contain considerable amounts of colored pigments which absorb a large part of the irradiated energy, which is thus unavailable for photochemical reactions, a comparatively large amount of sensitizer must be added. Frequently, as a result, amounts up to 5 to 10 weight percent of the reaction mixture, i.e., the printing dye, must be employed whereas otherwise in the synthetic resin industry, in the absence of coloring additives, only 1 to 2 weight percent are often completely sufficient. Usually even this relatively low concentration can barely be achieved with the sparingly soluble known sensitizers. In the case of the much higher concentrations necessary in printing dyes, the known sensitizers frequently partially crystallize out. In addition to the fact that the proportions which have crystallized out no longer act sensitizingly, after some time, the resultant crystallites also damage the printing plates consisting of relatively soft materials.
Acetophenone and derivatives thereof have also been suggested and used as photosensitizers, especially for photochemical cyclo-additions, [e.g., K. GOLLNICK "Type II Photooxidation Reactions" in Advances in Photochemistry, Vol. 6, pages 1-122 (Interscience Publishers, New York 1966, Editors: A. N. Noyes, G. S. Hammond and J. N. Pitts)]. The results obtained with compounds of this class, especially the quantum yields of the photochemical reactions sensitized therewith, are mostly markedly poorer than, for example, the results obtained with benzophenone.